1. Field of the Invention
The present invention relates to a method of forming satisfactory composite images free from fog.
2. Description of the Prior Art
In recent years, copying methods have been proposed of producing composite images by forming an image by exposing an original to light in the usual manner and further resorting to so-called writing with use of an OFT or laser. For example, Published Unexamined Japanese Patent Application SHO 57-8553 discloses such a method. In the first step of this method, a photoconductive member is charged by a corona charger to a surface potential Vs as seen in FIG. 1a. Subsequently in the second step, the photoconductive member thus charged is exposed to the optical image of a usual positive original as shown in FIG. 1b, with the result that the potential of the resulting image area remains approximately at the same level as Vs, while the potential of the nonimage area attenuates to VL. In the next third step, an OFT or laser is used for exposing the nonimage area to a negative image, whereupon the potential VL attenuates to VLL in corresponding relation to the image area as shown in FIG. 1c, whereby a composite latent electrostatic image is formed. The composite image is thereafter developed in the fourth step wherein the developing bias voltage is set approximately to VL. Consequently toners opposite to each other in polarity are deposited on the component latent images individually as shown in FIG. 1d.
To obtain sharp composite images free from fog by the above copying method, it is critical that the potentials Vs, VL and VLL be stable. Of these, the potential Vs is easily available with good stability at all times, for example, with use of a scorotron charger. Further the potential VLL is available also with good stability when the negative image is exposed to a sufficient amount of light. However, the intermediate potential VL between Vs and VLL is frequently unstable because the sensitivity of the photoconductive member varies from member to member and because of the temperature dependence of the sensitivity, variations in the amount of light for exposing the positive image, etc. Consequently difficulty is encountered in setting the developing bias voltage to the proper value. Meanwhile, the above publication SHO 57-8553 discloses that the potential VL is detected for controlling the amount of light for exposing the positive image, but this requires a complex device, and when the photoconductive light has irregularities in sensitivity longitudinally or circumferentially thereof, it is impossible to assure stable potentials, permitting fogging locally.
Further according to the above copying method, the latent images represented by Vs and VLL are individually developed at the same time by normal and reversal processes, which nevertheless have the drawback that fog is liable to occur and that the use of two kinds of toners for development disturbs the triboelectric characteristics thereof to result in fog. Moreover, depending on the purpose of edition, discrimination or the like, it often becomes necessary to develop the latent images with toners of different colors, but when the images are developed in two colors by a single developing device, the developing conditions, which are complicated, are liable to permit fogging, mingling of colors, etc. It is therefore best to develop the latent images by separate developing devices individually. However, experiments have shown that when developing the first latent image, i.e., the positive latent image represented by the above potential Vs, deposition of toner occurs in the form of contours along the intermediate potential (VL) edge portion of the negative latent image of VLL as shown in FIG. 2. The same is true of the case wherein the negative latent image of VLL is developed first, such that the toner is deposited in the form of contours along the intermediate potential (VL) portion of the positive latent image by an edging effect. Thus it is impossible to obtain flawless copy images.